The present invention generally relates to inductors and more particularly, to a noise suppression inductor for cancelling noises of a plurality of different frequency bands simultaneously.
In a known noise suppression inductor, as shown in FIG. 1, identical coils 3 and 4 are, respectively, wound, in such directions that magnetic fluxes of the coils 3 and 4 eliminate each other, around two portions of a bobbin 2 divided equally by a flange 1 and the bobbin 2 is fitted around one leg portion of each of a pair of U-shaped cores 5 and 6 which are butted against each other so as to constitute a core 7 of a closed magnetic circuit. The U-shaped cores 5 and 6 are secured by a spring plate 8. In the known noise suppression inductor of the above described arrangement, the coils 3 and 4 exhibit insertion loss characteristics having a single self-resonance frequency f0 which is determined by the inductance of each of the coils 3 and 4 and the stray capacity produced at each of the coils 3 and 4 as shown in FIG. 2.
Recently, in out-pulse dialling telephone sets such as a multifunctional telephone set, two kinds of noises having different frequency bands, for example, one noise of radio wave AM (amplitude modulation) radio broadcasting frequency band ranging approximately from 525 to 1605 KHz and the other noise radio wave of FM (frequency modulation) radio broadcasting frequency band and low channel (channels 1 to 3) TV broadcasting frequency band ranging approximately from 76 to 108 MHz, pose a problem.
Even if the known noise suppression inductor of the above described arrangement is used for cancelling such two kinds of noises having different frequency bands, the known noise suppression inductor having the single self-resonance frequency is merely capable of cancelling the noise of one frequency band. The noises form two different frequency bands cannot be cancelled effectively by the known noise suppression inductor simultaneously. Therefore, in order to effectively cancel such two kinds of noises from different frequency bands, two known noise suppression inductors having self-resonance frequencies falling respectively in the frequency bands of the noises to be cancelled are required to be used. Thus, the known noise suppression inductor has such drawbacks that due to increase of the number of the known noise suppression inductors required therefore cost for cancelling the noises rises and mounting of the known noise suppression inductor becomes troublesome.